LSI systems typically utilize active collector drivers which switch to down levels very fast because of the required DC drive to hold a given T.sup.2 L down level. Referring to FIG. 1, the transition curve for such a typical driver DVR is shown with a dotted line curve for di/dt (rate of change of output current). The curves demonstrate the high level of di/dt and I.sub.peak current for I.sub.0. The voltage induced in the on-chip stripe inductance L can be represented as V.sub.L =Ldi/dt. If a significant number of drivers (n) are switched at once, this value of V.sub.L, if high enough, may cause other drivers to falsely switch. Moreover, the total "on-chip" voltage use is significant because it limits the "on-chip" noise margin.
Within the prior art, techniques to limit the Ldi/dt voltage on an LSI chip have employed straddle capacitors. While such capacitors are effective to limit this induced voltage, a heavy penalty is incurred in terms of the usable chip area required for such elements. Typically, straddle capacitors require an area of approximately 10 mil.sup.2. Accordingly, an ongoing requirement exists to preserve the DC drive capability while limiting the Ldi/dt voltage yet requiring only a minimum amount of chip area on the LSI. The use of straddle or feedback capacitors is therefore not effective to satisfy these diverse requirements.
Given the existance of the stripe inductance L as a physical property, the on-chip wires and the inexorable increase in the number of drivers switching as logic chip density increases, the only effective way to limit total "on-chip" voltage is by control of di/dt. However, the prior art has not utilized this approach, preferring conventional capacitor techniques. As mentioned, such an approach may be satisfactory where chip area is no concern. But, in contemporary LSI devices, this is a primary aspect of design.
U.S. Pat. No. 3,867,649 shows a feedback capacitor circuit for a pair of identical drivers driving an MOS device. This patent shows the prior art use of a feedback capacitance 52. In operation, when the output of a particular driver falls, a current equal to CdV/dt is developed in the feedback capacitance 52. This current flow is utilized to divert current from the base of the transistor 48 and additionally, to slow down the fall time when the output is changing between levels, diodes 62 and 64 are utilized with the voltage divider resistors 70 and 72 to provide a controlled unidirectional feedback. Accordingly, spiking of the output is limited by diverting current from the base of the transistor 48.
Distinct from capacitance techniques, the consideration of active feedback circuit potentially offers one technique for meeting these diverse criteria.
U.S. Pat. No. 3,529,184 shows in FIG. 2 a modification of a Schmitt Trigger Circuit having an input voltage e.sub.n, and a transistor 14 with its emitter coupled to ground via emitter coupled resistor 20. An output, normally conductive, transistor 22 is coupled in a common emitter fashion to transistor 14. The emitter coupled resistor 20, in common with both transistor emitters, provides positive feedback resulting in the well known triggering action of the circuit. In order to overcome the inherent voltage drop across resistor 20 when transistor 22 is conductive, the '184 patent provides a feedback transistor 34. The emitters of output transistor 22 and feedback transistor 34 are connected to ground so that when the transistor 22 becomes fully conductive, the voltage at the collector falls substantially to ground potential. Accordingly, in the Schmitt Trigger Circuit of FIG. 2 of this patent, the base line voltage is substantially nearer to zero than is the case in conventional circuitry. U.S. Pat. No. 3,571,625 shows a pulse amplifier circuit having a feedback transistor 135 active only during transitions of the input pulses. The base-emitter diode of transistor 135 prevents any feedback through the series-connected resistors during application of the constant value voltage portion of the input voltage. However, during transitions in the signal applied to transistor 115 saturates the base-emitter junction of that transistor, transistor 125 is substantially cut off. A positive voltage at collector 127 forward biases the base-emitter diode of transistor 135 and current flows through resistors 131 and 133 enhancing the conduction of the transistor 115. Accordingly, positive feedback occurs only during pulse transitions.
Other patents specifically considered but not deemed to be pertinent include U.S. Pat. Nos. 3,555,306; 3,824,408; 3,971,961; 3,978,347; and 4,167,682. While the prior art shows several types of active switching devices, they either relate to the use of feedback capacitors as in the case of the '649 patent thereby incurring the heavy penalty of requiring large amounts of chip area or, do not minimize propagation delays in the switching of multiple drivers.